In recent years, diglycerides have been found to have an obesity preventing effect, a weight-gain controlling effect, and the like (PCT International Publication Pamphlet No. WO 99/48378, JP-A-04-300826, JP-A-10-176181), and their addition to various foods is being attempted.
It has been reported that the use of a glyceride mixture, which contains diglycerides at high concentration, as an oil phase can provide an edible oil-in-water emulsion composition having a rich fatty mouthfeel and a good taste and flavor even when the amount of fat is decreased (JP-A-03-008431).
It has also been reported that a stable oil-in-water emulsion composition can be obtained by using lysophospholipids as much as 15% or more (in terms of phosphorus amount) based on the total phospholipids in an acidic oil-in-water emulsion composition (JP-A-2001-000138). When a high-diglyceride oil or fat is used as an oil phase in an acidic oil-in-water emulsion composition, however, demulsification may take place under refrigerated conditions (−5° C. to 5° C.) to develop oil-water separation depending on the kind of fatty acids making up an oil or fat as a raw material. As a cause of such a phenomenon, it was considered that, as diglycerides have higher melting point than triglycerides, some of the diglycerides crystallize under refrigerated conditions. With a view to making an improvement in the low-temperature resistance under refrigerated conditions (−5° C. to 5° C.), an investigation was made into emulsifiers useful as crystallization inhibitors for oil and fat, and led to a report indicating that a particular range of emulsifiers are effective as crystallization inhibitors (JP-A-2002-176952)
As techniques for inhibiting crystallization of oil or fat composed primarily of triglycerides, on the other hand, it was already known to use a mixture of polyglycerol fatty acid esters containing a predominant proportion of saturated fatty acids esters (U.S. Pat. No. 2,266,591, JP-A-10-313820, JP-A-63-063343) or to use a mixture of polyglycerol fatty acid esters containing a large proportion of erucic acid esters (JP-A-2004-248518, JP-A-2002-212587). Plant sterols and plant sterol esters are also known to have tendency to crystallize in oil or fat when they are dissolved in the oil or fat, and therefore, methods have been reported to inhibit their crystallization (JP-A-2003-226890, JP-A-2003-160795) There are also a technique to prepare an acidic oil-in-water emulsion with diglycerides contained at low concentration by using enzyme-treated yolk as a method for stabilizing an acidic oil-in-water emulsion containing a plant sterol (JP-A-2002-171931, JP-A-2002-206100), and also a technique relating to the prevention of demulsification of an oil-in-water emulsion composition under vibrations applied during transportation by adding a plant sterol to the oil-in-water emulsion composition (PCT International Publication Pamphlet No. WO 03/047359), etc.